Electrically commutated, brushless direct current (dc) motors (spindle motors) are inductive motors used in a wide variety of commercial applications. One common application of such motors is in data storage devices, which use the motors to rotate one or more axially aligned data recording discs at a constant high speed (such as 10,000 revolutions per minute, rpm). As the discs are rotated, data transducing heads are controllably moved across the disc surfaces to access concentric tracks to which data are stored.
One type of spindle motor incorporates a stationary stator portion which supports multiple phase windings which are electrically connected in a star or delta configuration. A rotatable rotor supports a corresponding array of permanent magnets adjacent the windings. The rotor is rotated by electrically commutating the phase windings in a selected sequence. During each commutation period, drive current is input to one phase, output from another phase, and the remaining phase(s) are held at high impedance.
Spindle motors can employ back electromotive force (bemf) detection to detect the electrical rotational state of the motor and to time the application of each new set of drive pulses. Acceleration of a spindle motor from rest generally includes determining the initial electrical rotational alignment of the rotor with the stator, applying short duration drive pulses to accelerate the rotor from rest to an intermediate velocity at which the motor generates sufficient bemf to allow reliable detection of the rotational position of the motor, and then electrically commutating the spindle motor using closed loop bemf detection circuitry to accelerate the rotor to the final operational velocity.
It is desirable to limit the maximum current applied to a spindle motor to prevent damage to the motor windings and associated control circuitry. Excessive current draw can also adversely affect the voltage output of a power supply used to supply power to the motor as well as to other electronic circuits. However, reducing the maximum available levels of drive current generally reduces torque and increases the time required to accelerate the motor to the final operational velocity.
Accordingly, there is a continued need for improvements in the art whereby a spindle motor can be reliably accelerated from rest to an operational velocity, and it is to such improvements that the present invention is directed.